1. Field of the Invention
The present invention relates to a rocking piston type compressor.
2. Related Art
A rocking piston type compressor includes a piston rod that reciprocates in a cylinder while rocking by means of a connecting rod connected to a crank shaft. In the rocking piston type compressor, a lip ring is provided at a leading end portion of the piston rod, and the lip ring creates a seal between the cylinder and the piston rod.
The lip ring, when continuing to be used, is affected by compression heat or pressing load against a cylinder wall surface thereby to deform, or becomes worn due to the continuous use, so that a seal performance lowers and compression efficiency lowers gradually.
In order to prevent the seal performance of the lip ring from lowering, JP-A-09-068279 discloses a structure in which a lip portion of a piston ring is urged toward a cylinder by an urging member, thereby to create a seal between a piston body and the cylinder.
The lip ring is fixed to the piston rod by a ring holding member, and a clearance is provided between the lip ring and the ring holding member. Therefore, when the rocking piston slides in a compression direction, a pressurized air enters between the lip ring and the ring holding member, and back pressure is applied to the lip ring from an inner periphery thereof, so that the seal performance of the lip ring can be enhanced.
However, in case that the back pressure is too high, a load is applied onto the lip ring due to rapid pressure variation and compression heat, and deformation and abrasion are produced, which causes decrease of the seal performance of the lip ring.
Further, in case that damage of the lip ring results from the deformation and the abrasion, the damage is frequently inflicted further on surrounding parts. Since a load on the lip ring becomes largest in a rocking direction of the piston rod, the lip ring is frequently damaged on a side in the rocking direction of the piston rod. If the lip ring is damaged in the rocking direction of the piston rod, the rocking piston rod comes into direct contact with the cylinder (the lip ring cannot absorb the impact), which causes damage of the piton rod and the cylinder, or damage/breakdown of a bearing or a pressure meter due to vibration resulted from the contact between the piston rod and the cylinder.